The present invention relates to a device for triggering a lighting device in a motor vehicle.
European Patent Application No. 0 935 404 describes a circuit for stabilizing the voltage for a lamp, i.e., for headlights on a motor vehicle. The lamp voltage is stabilized at a reduced power loss by the fact that an additional time-delay switch controllable by a trigger circuit is included in the lamp circuit; when the power supply voltage is lower than the predefined nominal operating voltage of the lamp, the trigger circuit keeps the time-delay switch switched on continuously; and at a higher power supply voltage than the nominal operating voltage of the lamp, the trigger circuit periodically switches the time-delay switch on and off, the ratio of the on time to the duration of the period corresponding to the square of the ratio of the nominal operating voltage to the applied power supply voltage.
It is an object of the present invention to further stabilize the lighting intensity of a lighting device arranged in a motor vehicle, even for different operating states of the vehicle.
The device according to the present invention for triggering a lighting device of a motor vehicle includes a control arrangement which generates at least one pulse width modulated control signal for at least one switching arrangement via which at least one lighting device may be supplied with electric power. The control arrangement receives a measure of the power supply voltage which supplies power at least to the lighting device for influencing the control signal as a function of the power supply voltage. The control arrangement determines the control signal as a function of at least one additional operating parameter of the vehicle in addition to the power supply voltage. Due to the variable adaptation of the pulse width modulated control signal, different operating situations of the motor vehicle may be taken into account in which, as experience has shown, light intensities which fluctuate to different extents may occur. The driving speed is available as one possible operating parameter. The battery is usually under the greatest load when the vehicle is standing still, so that stabilization has priority. However, the brightness of the headlights need not be maximized when the vehicle is stationary. Driving speed as a possible operating parameter is taken into account in determining the control signal. Furthermore, flickering of the lights caused by other electric loads being turned on or off is hardly noticeable while driving. Thus, at high speeds, the stabilization function is not so strongly emphasized as adequate brightness of the lamp, i.e., the headlights.
In an example embodiment, the control arrangement specifies a maximum pulse width as a control signal when the power supply voltage drops below a lower limit. The lower limit is variable according to the present invention as a function of at least one operating parameter of the vehicle. The calculation procedure for determining the pulse width is stored in the control arrangement as a function of the lower limit value. Only this one value need be varied as a function of the operating parameter in order to take into account the prevailing operating situation of the vehicle in setting the control signal. The corresponding algorithm thus has a relatively simply structure.
In an example embodiment, the control signal may depend on the charge state of the automotive battery. The charge state is determined, for example, in conjunction with the power supply voltage before starting the vehicle and is used for setting the new lower limit value, for example. In the case of a high power supply voltage, indicating a fully charged automotive battery, the lower limit value may be set higher, because stabilization then plays a subordinate role. When the battery is relatively low, however, the tendency of the lighting device to flicker is much greater. Therefore, fluctuations in power supply voltage in a larger stabilization range are taken into account in setting the control signal.
In an example embodiment, the control arrangement specifies a minimum pulse width as the control signal when the power supply voltage exceeds an upper limit value. The minimum pulse width is variable as a function of the operating parameter. Due to this definition of the minimum pulse width, the stabilization range of the control arrangement may also be adapted to the prevailing operating situation. With an increase in driving speed, the minimum pulse width is increased for adaptation of the stabilization range. In addition, this provides overvoltage protection and lengthens the lifetime of the battery.
An example embodiment of the device according to the present invention is illustrated in the drawings and is described below.